LLW Solidification Incement-Effect of Dilution
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ABSTRACT A simulated Low Level nuclear Waste (LLW) solution was tested for long term solidification in a cement-based matrix. The waste is characterized by high pH and high concentrations of sodium, aluminum, nitrate, nitrite, phosphate and carbonate. The effect of diluting the waste with additional water was studied. The cementitious matrix was composed of cement, fly-ash and clay (21%, 68% and 11% respectively) with high solution to solid ratio (1 liter / 1 kg.). Mixes were prepared at 45'C and cured at 90'C for 28 days. Maximum 28 day compressive strengths and early age heat evolution were achieved by diluting the LLW solution to approximately 67% of its original concentration. More dilution led to a lower heat evolution and compressive strength. No dilution was found to give lower compressive strength, and a heat evolution that was delayed, and lower in intensity. XRD spectra showed formation of zeolites and tobermorite at the higher concentrations (67, 85, and 100% of the concentration of the undiluted simulated LLW), with a change from Na-P1 zeolite for 67% of the undiluted concentration to a sodalite at 100%. SEM observations showed a porous system for the low and high dilution rates but a less porous one for an intermediate level of dilution. INTRODUCTION Solidification in a cement matrix has been found to be an effective method for the disposal of low level nuclear waste (LLW) [1,2,3]. Many of the LLW's consist of solutions, characterized by high concentrations of sodium, aluminum, nitrate, nitrite, phosphate and carbonate ions. In this paper the solidification of a highly alkaline waste is considered. Cementitious systems containing high levels of fly-ash or granulated blast furnace slag have been found to be suitable for this purpose [4,5,6]; replacing much of the Portland cement with less reactive mineral admixtures reduces the adiabatic heat output.
EXPERIMENTAL PROCEDURE The low level waste was simulated by 5 species as shown in Table 1. A series of experiments were performed in which this solution was diluted to concentrations of 85%, 67%, 33% and 5.5% of the tabulated concentrations. The solubility of the sodium phosphate is strongly affected by temperature (its solubility in water ranges from 15g/l at 00 C to 1570g/1 at
209 Mat. Res. Soc. Symp. Proc. Vol. 370 01995 Materials Research Society
Table I: Composition of the simulated waste before dilution. Compound NaOH AI(NO3) 3 .9H20 Na 3PO4 .12H 20 NaNO2 Na2CO3
Amount,
9/1
M
74.85 128.0 74.4 36.9 36.2
1.87 0.34 0.20 0.54 0.34
Table II: Oxide composition (% weight) of dry materials.
70*C), so the solution was kept at 45*C in all cases to prevent precipitation of sodium phosphate. The solid materials were a type I/Il Portland cement (Ash Grove, Durkee, OR), a type F flyash (Centralia, Ross Sand and Gravel Co., Portland, OR) and an attapulgite clay (Engelhard, Iselin, NJ). The oxide composition of the solid materials is presented in Table II. Proportions by weight of 21% cement, 68% fly ash, and 11% clay were blended by end over end rotation
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